Contrast adjustment and gamma correction are two standard and effective image enhancement approaches. They work well for pictures of natural scenes having abundant colors. However, for document images captured either by scanners or cameras, these two methods often fail because:
1. Document images, especially images captured from black-white documents often have non-continuous color changes. These non-continuous color changes make the image very sensitive to the enhancement approach. Once enhancement fails, the image quality will be significantly degraded which will significantly reduce the quality of the OCR (Optical Character Recognition) result.
2. On a regular document image, the amount of foreground pixels (usually text) is much less than the amount of background pixels (white paper). It is almost impossible to set a fixed threshold to determine the cutting point of background versus foreground as in the standard contrast adjustment approach. On the other hand, the standard gamma correction approach changes the brightness of both foreground and background which makes the image a little blurry. Such a change might be desirable for images with natural scenes but definitely is not desirable for document images.
3. Because of the skewing of documents during scanning, the scanning background (often very dark) will affect the results of both contrast adjustment and gamma correction.
4. Wrinkles of physical documents often affect the scanned image quality. Neither standard contrast adjustment nor gamma correction is able to remove these wrinkles. The wrinkles thus appear as noise on the scanned images.
FIG. 1 illustrates an example of contrast adjustment and gamma correction result for an original image with a natural scene (FIG. 1(a)). It is easy to tell that contrast adjustment makes the image sharper (FIG. 1(b)) while gamma correction makes the image brighter (of course it can be made darker) (FIG. 1(c)). In contrast, FIG. 2 shows the correction results for a receipt image with the same parameters as those used in FIG. 1 (e.g., head and tail contrast adjustment percentage 10% and gamma value 0.5). Although contrast adjustment makes the image sharper, it also makes the noise sharper which will affect the OCR result. Also, although gamma correction makes the image brighter, it also makes the foreground blurry which is not desirable for OCR either. FIG. 2(d) shows the desired enhancement result made possible in accordance with the invention. As illustrated, the foreground text is maintained while the background is brightened. Also, wrinkles that appeared on the original image are removed.
A technique for bringing about such enhancement results is desired and is described herein for the enhancement of color images.